Field of Invention
The present invention relates to the field of aerospace turbomachines and, more particularly, that of pressurizing the internal chambers of such turbomachines.
Description of Related Art
Modern turbomachines generally take the form of an assembly of modules comprising either moving parts or stationary parts. Starting from the upstream end, they first of all comprise one or more compressor modules, arranged in series, which compress air sucked into an air intake. The air is then passed into a combustion chamber where it is mixed with fuel and burned. The combustion gases pass through one or more turbine modules which drive the one or more compressors. The gases are finally ejected either through a nozzle to provide a propulsive force or through a free turbine to provide power on a transmission shaft.
The rotating parts, such as the one or more rotating shafts, the one or more compressors and the one or more turbines, are carried by structural parts by means of bearings which are enclosed in chambers allowing the bearings to be lubricated and cooled. Turbomachines generally comprise two lubrication chambers, one located in the forward region which encloses the compressor-side bearings and one located in the rear region which encloses the turbine-side bearings. These chambers consist of a collection of moving and stationary walls, between which are arranged labyrinth-type devices in order to ensure the necessary sealing therebetween.
The stationary part of the forward chamber is made up of elements of a structural part termed the intermediate frame, while the rear chamber is made up of elements of a second structural part termed the exhaust frame. Examples of such chambers are represented in FIGS. 1 and 2. These structural parts support the bearings which in turn support the moving parts of the turbomachine.
In order to ensure that the oil is maintained inside the lubrication chambers, these are generally kept at a higher pressure than the surrounding spaces. To this end, pressurized air is injected into these chambers through orifices designed for this purpose. At the outlet of the chamber, an air/oil mixture is collected, the constituents of which are then separated by an oil separator-type device so that the oil can be sent to the ad hoc reservoir and the pressurization air can be vented to the outside.
In the prior art, the air for pressurizing the chambers is bled from downstream of a compressor stage, generally downstream of the low-pressure (LP) compressor. In existing engines, this pressure is sufficiently high to achieve the desired overpressure, without the air bled therefrom being at too high a temperature. In modern engines, where compression ratios in the compressors are ever higher, the temperature of the air leaving the LP compressor is relatively high. It follows that the temperature of the air entering the second chamber is too high, due to the heat energy imparted to it during its passage through the engine, this path taking it alongside the hot parts of the engine. It would then not be able to carry out its task of cooling the oil of the chamber, the temperature of which could then exceed 200° C. in certain working phases, which is not within acceptable limits.
One possible solution might be to bleed air further upstream than the exit from the LP compressor, but the pressure of the air bled would then not be high enough, especially at low engine speeds and when running on the ground, to adequately supply the chambers. There would then be the risk of inadequate air flow rate, or even of the air for cooling the chambers reversing its flow direction.
Another solution might be to fit an additional heat exchanger of the air/oil or air/fuel type and/or a return line for returning the fuel to the tank in order to increase the oil flow rate, and thus its cooling potential, at low engine speeds. However, such solutions are complicated to implement and give rise to additional weight.